Adobe licenses its H.264 encoder from Mainconcept, which doesn't do that well at low bitrates. x264 is pretty much the frontier when it comes at low size output for a given quality target, or quality for a given bitrate target. x264 is what's used by platforms like Youtube / Vimeo ..etc to encode user videos.
One thing you could try is to increase the ...
it looks like the Video is encoded in 4000 kbit/s, instead of a Rate Factor (RF).
But that isn't the issue here. The issue is: while 264 does support lossless encoding, it not widely compatible! (you would do this by setting the color format to hi444 and -cq 0 (Thanks to Gyan for pointing this out)
If you want the best quality, but also the largest file, you ...
I avoid AME and use x264 via ffmpeg for H.264 encoding. From Premiere I prefer to output a temporary lossless compressed format as the intermediate, such as the free and open-source Ut video, instead of DNxHD/DNxHR/ProRes. This avoids any generation loss (minor as it may be with ProRes/DNxHD, but still technically present as they are not lossless). Also, I'm ...
A one frame H264 stream would have to consist of an intra-coded frame, so there is a size floor in effect.
This ffmpeg command produces a 1.7 kiB file
ffmpeg -f lavfi -i color=blue:s=1280x720 -vframes 1 out.mp4
You can save ~200 bytes by lowering the resolution.
I was able to find an answer. https://stackoverflow.com/a/14308561/941493.
In the nutshell, CRF calculation is also affected by preset so the same CRF number is not the same with different presets. Based on some developer difference can be mostly ignored.